This feature is turned on by accident mostly, however it can be useful if you do not wish to be disturbed. As a receptionist you should never use this feature. It can be toggled on and off using the DND Hard key or soft key (if you have one) alternatively it can be changed under menu -> features -> DND on/off. If DND is active, you will not receive any calls to that phone. You will see when it is active, there will be a DND symbol on the top right of the screen.
Step 1 select Settings:
Step 2 Select basic Settings:
Step 3 Select DND:
This is what the screen looks like when DND is active.